Recently, various flat panel displays have been developed, and in particular, an organic electroluminescence display device (hereinafter, referred to as an organic EL display device) including an organic electroluminescence element (hereinafter, referred to as an organic EL element) has garnered great attention as an excellent flat panel display in terms of being able to achieve reduced power consumption, thinning, higher picture quality, etc.
FIG. 14A is a view describing injection properties and transport properties of an electron carrier in a known organic EL element using an LiF layer as an electron injecting layer.
As illustrated in FIG. 14A, known organic EL elements using an LiF layer as the electron injecting layer include a configuration, wherein an aluminum (Al) layer as a negative electrode, an LiF layer as the electron injecting layer, and an organic electron transport layer (ETL layer) are sequentially layered. In addition, the aluminum (Al) layer as the negative electrode and the LiF layer as the electron injecting layer are bonded by interface diffusion between Al and Li.
In accordance with the above-mentioned configuration, because there is almost no difference between the work function of the aluminum (Al) layer and the lowest unoccupied molecular orbital (LUMO) level of the LiF layer, there is no electron injection barrier, and good injection properties of an electron carrier can be obtained. Because of the small difference between the work function of the LiF layer and the lowest unoccupied molecular orbital (LUMO) level of an organic electron transport layer (ETL layer), good transport properties of an electron carrier can also be obtained.
However, because the LiF used as the electron injecting layer is a material susceptible to degradation under the influence of moisture and oxygen in the atmosphere, strict sealing for blocking moisture and oxygen, etc. in the atmosphere is required upon manufacturing an organic EL display device including an organic EL element with the LiF layer as the electron injecting layer. Such a strict sealing requirement has been a major factor to hinder achieving low costs and flexibility of the organic EL display device.
Accordingly, PTL 1 describes the use of a layer, including metal oxide with a stoichiometric ratio (composition ratio) of In:Ga:Zn:O=1:1:1:4 (hereinafter, referred to as an InGaZnO layer), which is stable in the atmosphere, and has a small work function, as the electron injecting layer.
While an electron injecting layer has been thus far described as one example, with regard to a hole injecting layer, an attempt has been made to use a layer including a metal oxide having a large work function as a hole injecting layer for improving problems, etc. occurring in the production process.
PTL 1 describes a configuration wherein a hole injecting layer including molybdenum trioxide with a single composition is formed between a positive electrode and a hole transport layer, and PTL 2 describes a configuration wherein a hole injecting layer including a mixture of molybdenum oxide and tungsten oxide with a single composition is formed between a positive electrode and an organic light emitting layer.